Failures in industrial oil systems have been associated with sludge and varnish formation. Sludge and varnish are insoluble materials formed as a result of either degradation reactions in the oil, contamination of oil or both. Explanations for the formation of sludge and varnish in these systems have varied but typically include the nature of the base oil, additive instability or degradation, bulk oil oxidation, electrostatic discharge and low electrical conductivity.
Much attention has been directed to the potential role of fluid electrification and electrostatic discharge as a prominent contributor to sludge and varnish formation in industrial oil systems. Electrostatic discharge is a form of localized thermal degradation. Electrostatic charge generation occurs in fluids systems as a result of internal molecular friction and electrical potential between the fluid and machine surfaces. The magnitude of the electrostatic charge within the oil depends on many factors and grounding of the machine itself has little impact toward mitigating electrostatic charge propagation. This is because the oils used are nonconductive and effectively self-insulate the charged fluid zones from grounded surfaces. Once a charge builds up in a working fluid zone, the subsequent static discharge may cause localized thermal oxidative oil degradation.
In the prior art, many antistatic additives have been suggested and used. U.S. Pat. No. 6,645,920 is directed to a lubricating oil composition for inhibiting rust and/or oxidation without formation of filter plugging deposits and sludge caused by acidic rust inhibitors in lubricating oil. The composition comprises an oxidation package, a rust inhibitor, a metal deactivator, an oil of lubricating viscosity and optionally other additives. The acidic rust inhibitor may be hydrocarbyl amine salts of hydrocarbyl aryl sulphonic acid. The oil of the invention may be natural or synthetic lubricating oils such as mineral, vegetable and animal oils and polymerized olefins, liquid esters and Fischer-Tropsch hydrocarbons.
U.S. Pat. No. 5,744,431 is directed to a lubricant composition used to prevent electrostatic charge build-up during operations of magnetic disk drives in computer system. One of the antistatic additives disclosed is STADIS 450, which was previously used as an antistatic additive in aviation fuels. The lubricating grease of the disclosed invention consists of light oil and a thickener, where the light oil consists of mineral oil, polyalphaolefins, diesters or aliphatic esters of polyol.
U.S. Pat. No. 5,940,246 is directed to a hydrobearing fluid for use in hydrodynamic bearing spindle motors for disc drive data storage devices. The hydrobearing fluid comprises an electrically non-conductive lubricant and an electrically conductive, non-metallic, non-magnetic additive. Lubricant base oils include mineral based hydrocarbons, synthetic hydrocarbons or esters. Additives include commercially available organic polymers such as a solution of a solvent (toluene, isopropyl alcohol, and other aromatic solvents C9-C16), dodecyl, benzene and sulfonic acid.
U.S. Pat. No. 6,335,310 is directed to a conductive lubricant for a fluid dynamic bearing to be assembled into a hard disk drive. The conductive lubricant comprises an ester based oil and an antistatic additive. The antistatic additive may be an alkyl aryl sulfonate, which is a salt of neutralization of alkyl benzene sulfonate and alkylamine.
Despite the advances in lubricant oil formulation technology, there remains a need for antistatic additives that effectively improve the electrical conductivity in lubricant compositions while maintaining the compositions dielectric strength and thereby reducing the formation of sludge and varnish in oil based lubricants.
It has been found that oil based lubricants having an electrical conductivity of at least 50 pS/m at 25° C. and a dielectric strength of at least 300 V/mil would significantly reduce electrostatic discharge in industrial oil systems thereby reducing the formation of sludge and varnish.